1. Field of the Invention
The present invention relates to a carrier for an electrophotographic developer and an electrophotographic developer containing the same. More particularly, the carrier of the present invention has an optimum resistance, is prevented from reducing its charging ability, and thereby provides an electrophotographic developer which has an extended service life, achieves high printing density, and causes no toner scattering nor fogging.
2. Description of the Related Art
It has been proposed to coat carrier particles for two-component developers with various resins so as to extend the service life of developers and to ensure high image quality as disclosed, e.g., in U.S. Pat. No. 3,526,533 and Japanese Patent Laid-Open No. 59-131944. Silicone resins, acrylic resins, fluoro/acrylic resins or like resins have high resistance, however, only to provide resin-coated carriers with increased resistance, which will cause such defects as an edge effect and an increase of charge quantity, failing to exhibit desired performance.
As a countermeasure, incorporation of an electrically conductive substance (conducting agent) such as carbon black into the resin coat of resin-coated carriers has been proposed as taught, e.g., in Japanese Patent Laid-Open Nos. 56-75659 and 54-7343. Carbon black, etc. dispersed in the resin coat acts to decrease the carrier resistance thereby improving developing properties and suppressing an increase of charge quantity.
However, as the amount of a conducting agent on the carrier surface increases, the carrier reduces its charging ability. It follows that toner particles with insufficient charges are produced, which may be scattered or cause image defects such as fog.
To overcome the problem, attempts have been made to improve carrier performance by providing a resin coat comprising two or more layers as disclosed in Japanese Patent Laid-Open Nos. 3-259157, 7-140723, 7-219281, 8-179570, and 8-146663. However, since the outer surface layer of the resin coat according to these methods contains no conducting agent, the resin coat has a high surface resistance to reduce the electrode effect of the carrier. The effect in suppressing an increase of charge quantity is also lessened, and the performance as expected cannot be obtained.
Japanese Patent Laid-Open No. 8-286429 proposes a carrier for a dry two-component developer, in which the resin coat has a double layer structure containing conductive carbon black in its inner layer and a white conducting agent in its outer layer. Because of the difference between the inner and the outer layers in kind of the conducting agent incorporated, the resin coated carrier has insufficient strength, that is, insufficient durability.
An object of the present invention is to provide a carrier which has an optimum resistance, is prevented from reducing its charging ability, and thereby provides an electrophotographic developer which has an extended service life, achieves high printing density, and causes no toner scattering nor fogging; and a developer containing the carrier.
As a result of extensive investigation, the present inventors have found that the above object is accomplished by a carrier coated with a double-layered resin coat having an outer layer and an inner layer each containing the respective amount of a conducting agent, the inner layer having a higher conductive agent content than the outer layer.
Based on the above finding, the present invention provides a carrier for an electrophotographic developer which comprises a carrier core and a resin coat having a conducting agent dispersed therein, the resin coat having an inner layer on the carrier core side and an outer layer coating the inner layer, wherein the inner layer contains 7 to 20% by weight of the conducting agent based on the resinous solid content of the inner layer, the outer layer contains 3 to 15% by weight of the conducting agent based on the resinous solid content of the outer layer, and the conducting agent content A (wt %) of the inner layer and the conducting agent content B (wt %) of the outer layer satisfies the relationship: B/A less than 1.0.
The present invention also provides an electrophotographic developer comprising the above-described carrier and a toner.
The carrier for an electrophotographic developer of the present invention has optimized resistance and charging ability so that reduction in image density due to an increase of charge quantity and reduction in charging ability are prevented. The developer containing the carrier of the invention has a prolonged service life, achieves a high printing density, and does not cause toner scattering or fog.
The carrier of the present invention has a resin coat comprising an insulating resin having dispersed therein a conducting agent. The resin coat has a double layer structure composed of an inner layer on the core side and an outer layer which coats the inner layer.
The insulating resin which can be used includes polyolefin resins, such as polyethylene, polypropylene, chlorinated polyethylene, and chlorosulfonated polyethylene; polyvinyl or polyvinylidene resins, such as polystyrene, acrylic resins (e.g., polymethyl methacrylate), polyacrylonitrile, polyvinyl acetate, polyvinyl alcohol, polyvinyl butyral, polyvinyl chloride, polyvinyl carbazole, polyvinyl ether, and polyvinyl ketone; vinyl chloride-vinyl acetate copolymers; styrene-acrylic acid copolymers; silicone resins, such as a straight silicone resin composed of an organosiloxane bond, or modified resins thereof (e.g., alkyd-, polyester-, epoxy- or polyurethane-modified silicone resins); fluororesins, such as polytetrafluoroethylene, polyvinyl fluoride, polyvinylidene fluoride, and polychlorotrifluoroethylene, and modified fluororesins; polyamide; polyester resins, such as polyethylene terephthalate; polyurethane; polycarbonate; amino resins, such as urea-formaldehyde resins; and epoxy resins. Of these resins, preferred are acrylic resins, silicone resins or modified silicone resins, and fluororesins or modified fluororesins. Silicone resins and modified silicone resins are particularly preferred.
The resins making the inner layer and the outer layer may be the same or different, but it is usually preferred to use the same resin.
A curing catalyst can be added to these resins. Curing catalysts suitable for silicone resins, for example, include octylic acid and quaternary ammonium salts, such as tetramethylammonium acetate.
The conducting agents include carbon black (e.g., KETJEN BLACK, furnace black or acetylene black) and white conducting agents, such as TiO2, ZnO2 and SnO2. Carbon black is particularly preferred. Of the white conducting agents TiO2 is preferred.
The conducting agent content in the inner layer is 7 to 20% by weight, preferably 10 to 15% by weight, based on the resinous solid content of that layer. The conducting agent content in the outer layer is 3 to 15% by weight, preferably 5 to 10% by weight, based on the resinous solid content of that layer. If the conducting agent content in the inner layer or the outer layer is less than 7% by weight or 3% by weight, respectively, the charge quantity increases to reduce the image density. If the conducting agent content in the inner layer or the outer layer exceeds 20% by weight or 15% by weight, respectively, the charging ability reduces to cause fog.
The conducting agent content in the inner layer, taken as A % by weight, and that in the outer layer, taken as B % by weight, should satisfy the relationship: B/A less than 1.0. B/A is preferably in the range of from 0.2 to 0.8. If B/Axe2x89xa71.0, fog occurs, and the toner is scattered to cause image defects in long-term use.
Conventionally known carriers can be used as a core material to be coated, such as iron powder, ferrite powder, and magnetite powder. Ferrite powder of various kinds is preferred because ferrite powder is easy to control as to the surface condition, shape, resistance, etc. which are influential on the characteristics of the carrier after being coated. The carrier particles preferably have an average particle size of 20 to 200 xcexcm.
The total coating weight of the resin on the core, inclusive of the inner and outer layers, is 0.03 to 5.0% by weight, preferably 0.05 to 3.0% by weight, based on the core. A coating weight less than 0.03% tends to fail to form a uniform coat on the carrier surface. A coating weight exceeding 5.0% forms a so thick resin coat that the coated carrier particles may agglomerate with each other, and it would be difficult to obtain uniform carrier particles.
Coating of the carrier core with the resin is usually conducted by a wet process comprising applying the resin as diluted with a solvent onto the surface of the core by dipping, spraying, brushing, kneading or a like technique and volatilizing the solvent. A dry process comprising coating the core with a powdered resin is also employable.
After coating, the coating layer can be baked, if desired, either by external heating or internal heating by means of, for example, a fixed bed or fluidized bed electric oven, a rotary kiln type electric oven, a burner oven, or a microwave oven. The baking temperature preferably ranges from 150 to 300xc2x0 C.
The resin-coated carrier according to the present invention is mixed with a toner to provide a two-component developer. The toner to be used comprises a binder resin having dispersed therein a colorant, a charge control agent, etc. Known black and color toners can be utilized.
While not limiting, the binder resin which can be used in the toner includes polystyrene, chloropolystyrene, a styrene-chlorostyrene copolymer, a styrene-acrylic ester copolymer, a styrene-methacrylic acid copolymer, a rosin-modified maleic acid resin, an epoxy resin, a polyester resin, a polyethylene resin, a polypropylene resin, and a polyurethane resin. These binder resins can be used either individually or as a mixture thereof.
The charge control agent which can be used in the toner is selected arbitrarily. Useful charge control agents for positively chargeable toners include nigrosine dyes and quaternary ammonium salts, and those for negatively chargeable toners include metallized monoazo dyes.
Any well-known dyes and/or pigments are useful as a colorant. Examples of suitable colorants are carbon black, Phthalocyanine Blue, Permanent Red, Chrome Yellow, and Phthalocyanine Green. The colorant is usually used in an amount of about 0.5 to 10 parts by weight per 100 parts by weight of the binder resin. External additives, such as fine silica powder and titania, can be added to the toner particles for improvement on fluidity and anti-agglomeration.
The method for preparing the toner is not particularly restricted. For example, a binder resin, a charge control agent and a colorant are dry blended thoroughly in a mixing machine, e.g., a Henschel mixer, and the blend is melt-kneaded in, e.g., a twin-screw extruder. After cooling, the mixture is ground, classified, and mixed with necessary external additives in a mixing machine, etc.
The carrier of the present invention has a resin coat to exhibit improved durability and contains in its resin coat a conducting agent for suppressing an increase of charge quantity and for decreasing the resistance to thereby improve developing properties. In addition to this structure, the resin coat of the carrier is composed of an inner layer and an outer layer different in content of the conducting agent. The conducting agent in the inner layer ensures retention of the optimum resistance of the carrier. The conducting agent content in the outer layer being made smaller than that in the inner layer, the carrier is prevented from reducing the charging ability. As a result, the carrier resistance can be optimized, and reduction in charging ability can be prevented, thereby prolonging the life of the developer and achieving high density printing. Further, the developer exhibits high performance free from toner scattering or fog.
The present invention will now be illustrated in greater detail with reference to Examples. Unless otherwise noted all the percents are by weight.